The present invention relates generally to light-emitting devices having organic electroluminescent material and, more particularly, to organic electroluminescent devices (xe2x80x9cOELDsxe2x80x9d), having improved energy efficiency and improved stability of optical output at low drive voltage.
Electroluminescent (xe2x80x9cELxe2x80x9d) devices, which may be classified as either organic or inorganic, are well known in graphic display and imaging art. EL devices have been produced in different shapes for many applications. Inorganic EL devices, however, typically suffer from a required high activation voltage and low brightness. On the other hand, OELDs, which have been developed more recently, offer the benefits of lower activation voltage and higher brightness in addition to simple manufacture, and, thus, the promise of more widespread applications.
An OELD is typically a thin film structure formed on a substrate such as glass or transparent plastic. A light-emitting layer of an organic EL material and optional adjacent semiconductor layers are sandwiched between a cathode and an anode. The semiconductor layers may be either hole (positive charge)-injecting or electron (negative charge)-injecting layers and also comprise organic materials. The material for the light-emitting layer may be selected from many organic EL materials. The light emitting organic layer may itself consist of multiple sublayers, each comprising a different organic EL material. State-of-the-art organic EL materials can emit electromagnetic (xe2x80x9cEMxe2x80x9d) radiation having narrow ranges of wavelengths in the visible spectrum. Unless specifically stated, the terms xe2x80x9cEM radiationxe2x80x9d and xe2x80x9clightxe2x80x9d are used interchangeably in this disclosure to mean generally radiation having wavelengths in the range from ultraviolet (xe2x80x9cUVxe2x80x9d) to mid-infrared (xe2x80x9cmid-IRxe2x80x9d) or, in other words, wavelengths in the range from about 300 nm to about 10 micrometer. Such EM emitted by OELDs can be further modified to produce light having a desired color. For example, modifying EM emitted by OELDs with one or more phosphors can produce white light used for general illumination. In this application, it would be advantageous to provide OELDs having a controllable brightness and improved energy efficiency for brightness control schemes. However, prior-art methods of brightness control by reducing the device drive voltage often lead to flickering when the drive voltage is substantially reduced.
Therefore, it is desirable to provide OELDs, the brightness of which is controllable, and-which have improved energy efficiency and improved stability of optical output when the drive voltage is reduced.
An organic electroluminescent device (xe2x80x9cOELDxe2x80x9d) of the present invention is capable of producing controllable brightness at improved energy efficiency and improved stability of optical output. The OELD comprises at least an organic EL material disposed between a pair of electrodes. The present invention also provides a light-emitting system that comprises at least an OELD and a power supply that provides a voltage to the OELD at high frequency.
In one aspect of the present invention, a plurality of OELDs is connected in series in the light-emitting system.
In another aspect of the present invention, a drive voltage is provided in pulses at a high frequency across the organic EL material of the OELD, and each of the drive voltage pulses has a maximum voltage value selected such that it produces an average brightness at the highest power efficiency when the OELD is activated. Power efficiency is defined as radiant energy output of the device per unit electrical energy input. The frequency, the duty cycle, or both are modified to provide a desired level of brightness in the circumstance.